ZX Spectrum Models

Here’s a brief overview of the various Spectrum PCB revisions, this section is split into three main areas:

Spectrum 16/48K Family. The original line produced by Sinclair.

Spectrum 128/Grey +2. First produced for Investronica of Spain, later released in UK. The grey +2 was Amstrad’s first machine and was heavily derived from the 128 architecture.

Spectrum +2A/+3. Cost reduced versions of the 128 architecture with some incompatibilities.

You can view a gallery of all the PCB revisions mentioned in this article at the bottom of the page.

16/48K Family

Issue 1

The first Spectrum ever produced, manufacture commencing in April 1982. This has hand drawn PCB traces and has a prototype ‘feel’, and was laid out by Stuart Honeyball, whose later employment included Miracle Systems.

It can only take 16K of RAM on the main PCB and required a daughterboard to supply the upper 32K, which plugs into two sockets at the top of the PCB. There are no IC designations on the silkscreen, and the lower RAM is not properly arranged in bit order as found on later issues. Can suffer from blue/yellow screen tinting which can usually be resolved by adjusting VR1/VR2, however other mods are sometimes required to completely cure this. The regulator and heatsink are situated at the bottom right of the PCB, the heatsink is a little undersized and such the machine can suffer from overheating problems.

Y, U, V and VID ‘straps’ are present on the PCB, if wire links are installed here then the relevant signals are carried out to the edge connector.

Visually, they can be most easily identified by their grey, ‘dead-flesh’ feel keymat and a small adjustment hole on the underside of the machine. If the 32K expansion daughterboard is fitted, its decoupling capacitors will be visible through the expansion port. The grey mat and/or hole are sometimes present on very early Issue 2’s however, and some very late Issue 1 machines may have a blue keymat.

Proceed with caution if buying a machine of unknown provenance without pictures of the PCB itself.

Approximately 16,000 of these machines were produced.

Issue 2

This followed the Issue 1 quite quickly in August of 1982, and was the first Spectrum issue to be laid out using CAD tools. It also has provision for the full 48K of RAM on the main PCB. Upper RAM is limited to Texas Instruments TI4532, but OKI 3732 can be used with a minor modifcation. IC designations are on the PCB silkscreen and the lower RAM is installed in order, bit 7 to bit 0 left to right. This issue is otherwise operationally identical to the Issue 1 circuit.

Issue 2 PCB’s contain probably the widest variety of ULA types – very early ones had 5C102E ULA’s (with cockroach), then 5C112E, and very late ones had 6C001E-5 and 6C001E-6 ULA’s.

Externally the most obvious change is to the keymat which is now blue in colour, although very early Issue 2’s may come with the Issue 1 keymat as mentioned above.

Approximately 500,000 Issue 2’s were produced.

Issue 3, 3B

Substantial changes were introduced with these PCB issues, notably the position of the regulator and heatsink (now positioned at the back of the PCB, next to the expansion slot), the colour and clock adjustment controls (TC1/TC2/VR1/VR2) are removed and replaced with an automatic circuit, the beeper is now transistor driven and is a 40ohm impedance part (instead of 200ohm on earlier models) and the ULA type is now 6C001E-6.

These modifications improved reliability somewhat, but unfortunately the 6C001E-6 ULA has a reputation of failing more often than other types.

The Y/U/V/VID straps present on Issue 1 and 2 machines are now removed and the signals are brought out directly to the edge connector.

Links on the PCB allow the official use of OKI as well as TI upper RAM for the first time.

The Issue 3B is a very minor revision of the Issue 3 design to improve the reliability of the DC-DC converter circuit.

Some 3B PCB’s were manufactured using SRBP materials and are yellow with blue silkscreen markings, others were manufactured by Samsung and have green solder mask on both sides of the PCB.

Over 3 million Issue 3 and 3B machines were sold, making it one of the most common Spectrums.

Issue 4A, 4B

This issue incorporates the 6C001E-7 ULA type, and also includes earlier service modifications to improve the DC-DC converter reliability directly on the PCB. Some timing issues were presumably discovered during layout, resulting in the /RAS line being routed through spare inverters to delay the signal (this is why Issue 4A and 4B machines won’t work with IC24 removed, unlike earlier models).

These PCB’s also include options to support the aborted US FCC certification of the Spectrum, such as a channel switch for the modulator, EM suppression coils on the DC input, case grounding points on the PCB etc.

Some of these PCB’s were also manufactured using SRBP materials, and are yellow in colour with blue silkscreen markings.

This is the first PCB issue to be widely available in the Spectrum+ case.

Issue 4S

Almost identical to the Issue 3B PCB (as it doesn’t route /RAS through IC24), but manufactured by Samsung and has green solder mask on both sides of the PCB, and extra solder pads so that alternative components can be fitted (notably the coil, TR4 and TR5). This can be found in both rubber key and Spectrum+ cases with serials starting with S01.

The keyboard membranes associated with these machines (in both rubber key and Plus variants) are of much better quality than standard, with many still working correctly today.

Issue 5

Detailed in the service manuals, only 1000 of these machines were thought to have been produced and none were known to still exist until recently. They have a layout error which requires a PCB modification and patch wiring to work, so presumably this is the reason for such low numbers.

They are also notable for being the first issue to incorporate the ZX8401/PCF1306P IC which was designed to replace the six 74-series logic IC’s on earlier issues. These in themselves caused further timing issues which required the inclusion of an extra 74LS04 on the PCB to fix.

The final 48K PCB issue, this seems to be a bugfixed version of the Issue 5, but also supports the AMI SAGA-1A ULA, which was intended to be an alternative to the Ferranti ULA but DRAM timing issues along with process variation problems led to its cancellation.

Only one Issue 6A machine with an AMI ULA is known to survive.

The Issue 6A also contains dedicated provision on the PCB for a reset switch to be connected, instead of the switch being installed across C27 as on earlier issues in Spectrum+ cases.

128/Grey +2 Family

Version 2

First version of the Spectrum 128 ‘Toastrack’, found in early Investronica 128 machines. These are presumably Samsung manufactured as they have solder mask on both PCB sides. The /CLK connection on the edge connector is not actually connected to the CPU clock signal, so this must be manually added if peripherals that use this signal (e.g. DivMMC) are to be used.

Despite the specification stating otherwise, RAM banks 1,3,5 and 7 are contended, and 0,2,4 and 6 are uncontended. This caused some issues as we’ll see later.

Version 2-1

Minor revision to correct some bugs on the Version 2 PCB, again found only in Investronica 128 machines. /CLK connection issue is still present on this revision.

Version 6K

The first mainstream UK-spec 128 machine, manufactured by Samsung. Its main distinguishing feature from the 6U is that it has green solder mask on both sides of the PCB, and uses similarly sourced components to that found in the 48K Issue 4S. /CLK signal on the edge connector is correctly connected on this and all subsequent issues.

However, there is an issue with the value of R137, which has the net result of causing the MC1488 keypad driver IC not to function properly due to out of spec voltage rails, and therefore a keypad will not work with this version. Since this was the first UK 128 version, this is most likely the reason that UK keypads were not sold (and today are extremely rare).

Changing R137 for a 15R resistor will allow the keypad to work on these machines. (credit to Ian Gledhill for this fix)

Version 6U

By far the most common variant of the Spectrum 128, this has solder mask on the PCB underside only, like earlier 48K machines. Some late machines feature soldered Amstrad 40056 ULA’s instead of the usual Ferranti 7K010E-5.

Version 9G

Final version of the Spectrum 128, mostly sold in European markets and can be distinguished by having a standard DB-9 RS232 connector instead of the usual BT631W socket.

Z70500 Issue 3

The first Amstrad produced Spectrum, this was reverse engineered from a Sinclair 128 machine by Amstrad engineers in quite a hurry in order to make the deadline for availability during Christmas 1986 (this effort actually commenced before the sale of Sinclair to Amstrad had been formally concluded). It was manufactured by Amstrad’s contract manufacturer (Orion) in Taiwan.

As such it is functionally identical to the Spectrum 128 machines, with the addition of onboard Sinclair-compatible joystick ports (controlled by and Amstrad 40057 IC, which is essentially a MT62001, found in the Interface 2).

The PCB quality is notably poor on the Amstrad-produced Sinclair PCB’s, requiring great care when replacing components to avoid lifting pads or tracks.

A slightly later revision of this board is marked ‘0500 Issue 3’, but apart from very minor layout changes there is no functional difference between the two.

Z70700 Issue 1

UK-built versions of the Grey +2 carry this PCB, which is functionally identical to the Z70500 but uses mostly axial capacitors and other UK-sourced components (notable, the Z70700 uses the PCF1306P where the Z70500 has the Amstrad 40058). It is speculated that this was produced to use up components that Amstrad inherited from ex-Sinclair suppliers as part of the takeover.

+2A/+3 Family

More in-depth technical information on the Amstrad produced +2A, +2B, +3 and +3B models is available here, however a short summary is contained below.

Z70830

These PCB’s are to be found in Spectrum +3 models (and some very early Spectrum +2A’s). They are a much cost reduced implementation of the Spectrum hardware, consolidating much of the functionality formerly accomplished by the ULA, various logic IC’s, PCF, Amstrad 40057, and HAL into a single QFP gate array (designated Amstrad 40077).

The DC-DC converter circuit present in the architecture from the 48K Issue 1 machine is removed, and instead the various voltage rails are fed directly from a new power supply with a 6-pin DIN connector instead of the traditional 2.1mm barrel connector.

The RAM architecture is changed to have just 4 RAM IC’s which are 64Kbit x 4, instead of 16 64Kbit x 1 IC’s in earlier 128 machines.

The split bus architecture is implemented internally to the gate array instead of externally using resistors as on all previous Spectrums, resulting in external peripherals not being able to drive the keyboard any longer.

RAM banks 0-3 are uncontended, and banks 4-7 are contended with this architecture which matches the original 128 specification, but which causes some software that was written against the actual 128/Grey +2 behaviour to run differently, with the most obvious effect being distorted beeper audio or sampled speech.

The audio circuitry has a major design error that results in the audio output being heavily distorted. This can be corrected by substituting some components for others with alternative values, and adding a couple of extra resistors. The modifications required are detailed here.

The main difference between +3 and +2A machines using this PCB is that on the +2A, IC’s and passive components that drive the FDD (uPD765A, SED etc) are not populated.

Z70833

This PCB was produced exclusively for later +2A and +2B machines, and are a cut down version of the Z70830 model that has the area that contains the FDD circuitry physically absent from the PCB.

Some Z70833 Issue 1 PCB’s have timing issues that mean that only certain types of 4464 RAM (either AMS or MT manufacture) can be used.

The faulty audio circuitry on the Z70830 PCB is mostly corrected.

The Z70833 Issue 4 PCB dates from 1990 and is the latest known revision of any Spectrum PCB.

Z70835

This is a relatively rare revision of the +3 PCB which incorporates fixed audio circuitry to the same standard as the Z70833 PCB. Unlike the Z70830, it omits the Tape header.

It is thought that this PCB revision only appeared in late model Spanish +3’s.

Issue 1 machine (001-011754) with 32K daughterboard installed.

Issue 1 32K RAM expansion board.

Issue 1 case. Note the grey keys which is the most obvious identifier for an Issue 1 machine.